Conventionally, as a method for modifying a surface of ferritic stainless steel, a high-temperature nitridation method is known, in which a ferritic stainless steel is heated at a nitriding temperature higher than a transformation temperature in an atmosphere of an inert gas containing nitrogen gas: N2 (e.g., Patent Document 1: JP 2006-316338 A corresponding to US 2007/0186999 A1). According to the high-temperature nitridation method, a nitrided layer can be formed on the surface of the ferritic stainless steel, and hardness and corrosion resistance of the ferritic stainless steel can be improved.
In Patent Document 1, it is described that a preferable range of the nitriding temperature is from 1150 to 1200° C. Moreover, in Patent Document 1, a removing process, in which a passive layer on a surface of a ferritic stainless steel is removed, is performed before the high-temperature nitridation process. The removing process is reduction treatment by using hydrogen gas.
The present inventors perform such high-temperature nitridation of a ferritic stainless steel at various nitriding temperatures. When the nitriding temperature is lower than 1110° C., a nitrided layer is not generated stably. Stable generation of the nitrided layer means that the nitrided layer is formed on all treated objects when the treated objects are nitrided in the same furnace at the same time. Therefore, incapability of the stable generation of the nitrided layer means that the nitrided layer is not formed on all or a part of the treated objects when the treated objects are nitrided in the same furnace at the same time. One reason why may be that removing of a passive layer existing on the surface of the ferritic stainless steel is insufficient, and nitrogen as a solute cannot mix stably with the surface of the ferritic stainless steel as a solvent to form a solid solution, at a nitriding temperature below 1100° C.
When the nitriding temperature is set higher than or equal to 1100° C., the nitrided layer can be formed stably. However, in this case, coarsening of crystal grain in a metallic structure may be occurred, and a lifetime of a furnace or a thermal treatment jig may be shortened.
In Patent Document 1, the removing process, in which the passive layer is removed by reduction treatment with hydrogen gas, is performed before the nitridation process. Thus, a device for introducing or discharging the hydrogen gas to or from the heating furnace may be necessary, and an equipment including the heating furnace may become complicated as a whole.
In Patent Document 1, when the product is produced in a large scale, the nitridation process is performed on each product or each batch. Before each nitridation process, it may be necessary to perform the removing process of the passive layer.